For example, in such a projection type projector apparatus, such as DLP (Digital Light Processing: Registered Trademark) that uses a DMD (Digital Micromirror Device (Registered Trademark)), an image with uniform and sufficient color rendering properties is projected onto a rectangle screen.
For this reason, the short arc type ultrahigh pressure mercury lamp, in which the mercury vapor pressure at the time of lighting turns into 150 or more atmospheric pressure, is adopted as a light source. Moreover, in a lamp that is used for an ultraviolet rays exposure or for lighting of an optical apparatus, a short arc type ultrahigh pressure mercury lamp, in which the mercury vapor pressure thereof at time of lighting turns into 100 or more atmospheric pressure, is adopted. A pair of electrodes is arranged so as to face each other at an interval of, for example, 2 mm or less in the arc tube of the ultrahigh pressure mercury lamp. The arc tube is made of, for example, silica glass, and mercury and halogen are enclosed in the arc tube. The main purpose of enclosing halogen in the arc tube is to form a halogen cycle in the arc tube and to suppress adherence of tungsten, which is electrode material, onto an inner wall of the arc tube. For example, Japanese Patent Application Publication Nos. 2005-063817, 2006-079986, 2000-231903 teach such an ultrahigh pressure mercury lamp, respectively.
FIG. 7 is an explanatory cross sectional view of the structure of the main part of an example of a conventional ultrahigh pressure mercury lamp. This ultrahigh pressure mercury lamp 80 is an alternating current lighting type system, in which lighting is carried out by impressing alternating current voltage. The ultrahigh pressure mercury lamp 80 has an arc tube 100 made of silica glass, wherein rod-like sealing portions 102 are formed at both ends of alight emission section 101. A pair of electrodes 90, which are respectively made of tungsten, is arranged so as to face each other in the light emission section 101 of the arc tube 100. Each of the electrodes 90 has a rod shape axis portion 91 whose base end portion is buried in and held by the sealing portion 102 of the arc tube 100. An approximately conic head portion 92 is integrally formed at the tip of each axis portion 91 through an approximately cylindrical body portion 93. A projection portion 92A is formed at the tip of this head portion 92. A conductive metallic foil (not shown in the figure) buried in each sealing portion 102 of the arc tube 100 is welded and connected to the base end portion of the axis portion 91 of the electrode 90. An external lead rod (not shown in the figure), which projects from an outer end of each sealing portion 102 of the arc tube 100, is connected to this conductive metallic foil. Moreover, in the example shown in the figure, a coil portion 94 is integrally formed on the circumference of each body portion 93 by melting a coil winded around the body portion 93. The coil portion 94 mainly heats the electrode 90 so as to help a temperature rise of the electrode 90 during a glow discharge period at lamp start-up time, thereby making easy transition from the glow discharge to arc discharge.
In such an ultrahigh pressure mercury lamp 80, there is a problem that a projection is formed at the tip of the electrode 90 during lighting, so that a flicker occurs. As a means for solving such a problem, there has been a unit for inserting intermittently alternating current of frequency lower than regular lighting frequency, in alternating current of regular lighting frequency, so as to supply it to the ultrahigh pressure mercury lamp from a power supply apparatus (refer to Japanese Patent Application Publication No. 2006-59790).